arm64: Introduce uaccess_{disable,enable} functionality based on TTBR0_EL1

This patch adds the uaccess macros/functions to disable access to user
space by setting TTBR0_EL1 to a reserved zeroed page. Since the value
written to TTBR0_EL1 must be a physical address, for simplicity this
patch introduces a reserved_ttbr0 page at a constant offset from
swapper_pg_dir. The uaccess_disable code uses the ttbr1_el1 value
adjusted by the reserved_ttbr0 offset.

Enabling access to user is done by restoring TTBR0_EL1 with the value
from the struct thread_info ttbr0 variable. Interrupts must be disabled
during the uaccess_ttbr0_enable code to ensure the atomicity of the
thread_info.ttbr0 read and TTBR0_EL1 write. This patch also moves the
get_thread_info asm macro from entry.S to assembler.h for reuse in the
uaccess_ttbr0_* macros.

Cc: Will Deacon <will.deacon@arm.com>
Cc: James Morse <james.morse@arm.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/kernel/cpufeature.c

@@ -47,6 +47,7 @@ unsigned int compat_elf_hwcap2 __read_mostly;
 #endif
 
 DECLARE_BITMAP(cpu_hwcaps, ARM64_NCAPS);
+EXPORT_SYMBOL(cpu_hwcaps);
 
 DEFINE_STATIC_KEY_ARRAY_FALSE(cpu_hwcap_keys, ARM64_NCAPS);
 EXPORT_SYMBOL(cpu_hwcap_keys);